Tough prison sentences for those involved in riots have been criticised as "disproportionate", as one man jailed for four years for inciting unrest on Facebook said he will appeal.

Lawyers and campaigners warned that the rush to send out a tough message and to make an example of those involved in violence was leading to "some very bad sentences".

The call came after Jordan Blackshaw, 20, and Perry Sutcliffe-Keenan, 22, were jailed for each setting up Facebook pages that encouraged people to riot. Even though no disorder occurred they were given four years each.

Blackshaw's solicitor Chris Johnson said his family was "shocked and upset" at the sentence and said he had been instructed to appeal against it.

Prime Minister David Cameron defended the punishment, saying: "What happened on our streets was absolutely appalling behaviour and to send a very clear message that it's wrong and won't be tolerated is what the criminal justice system should be doing.

"They decided in that court to send a tough sentence, send a tough message and I think it's very good that courts are able to do that."

Former party leader Sir Menzies Campbell said: "With all due deference to the Prime Minister, politicians should not be either cheering nor booing in the matter of sentencing," he said.

"It is an important part of our constitutional principles that political influence is not directed at the judicial system."

Leading criminal barrister John Cooper QC warned that judges and magistrates had a duty "not to be influenced by angry Britain", describing some of the sentences handed down already as "disproportionate and somewhat hysterical".